The recent advent of combinatorial libraries composed of tens to hundreds of millions of peptides and nonpeptides offers an unparalleled opportunity to identify novel, highly receptor-specific ligands for the known opioid receptor subtypes. We propose to use this novel approach for the identification of highly selective ligands, both agonist and antagonist, for the three opioid receptors mu, delta, and kappa. These three opioid receptors all mediate analgesia; however, the type of pain inhibited, as well as their secondary functions, have been shown to differ. Differences in receptor functions encourage the search for drugs that produce analgesia without the known deleterious side effects of morphine and other opiate analgesic agents. Opioid agonists and antagonists with high selectivity for each of the different opioid receptors (mu, delta, kappa) and their subtypes are required for the study of opioid pharmacology, and can also serve as potential pharmaceutical agents or leads for drug development. In spite of improvements in specificity and activity of naturally-occurring opioids, the time and cost for the synthesis of large numbers of peptides remain limiting factors. This laboratory has pioneered a number of the seminal advances in combinatorial technology and has been actively involved in their use in the opioid field for the past three years. Two approaches for the identification of individual active compounds from soluble combinatorial libraries have been developed by the principal investigator's laboratory. The first approach, termed a synthetic peptide combinatorial library (SPCL), uses an iterative selection and enhancement process to identify individual peptides. The second approach, termed a Positional Scanning SPCL (PS-SPCL), enables the potential identification of individual compounds of interest directly from the screening data. The feasibility of both of these approaches has been verified using case studies. Chemically diverse libraries, each of which contains tens of millions of individual compounds which can be composed of peptides made of either L-, D- or unnatural amino acids, dimeric or cyclized peptides, as well as similar numbers of non-peptide compounds, will be screened in radioreceptor assays specific for each of the three opioid receptor types, mu, delta, and kappa. Selective compounds identified from the libraries will be tested for agonist or antagonist activities in the guinea-pig ileum and mouse vas deferens bioassays. Highly active and selective agonists and antagonists will also be tested in standard in vivo assays.